A touchstone is a hard black stone with a smooth surface that can be used to test gold or silver for purity. The gold or silver is scratched or streaked onto the surface of the touchstone and then compared to streaks of known alloys. Testing needles or touch needles can be purchased with the points tipped with precious metal alloys of known purity for the purpose of making the comparison. The stone is generally a chert (Novaculite or Arkansas stone), but sometimes jasper is used.
Another method of alloy testing with a touchstone is to make a scratch with the jewelry item on the stone and applying an acid on the streak. Depending on whether the streak disappears or not, the alloy can be determined. One must make sure that not only the surface of the item is tested as it could be gilded. This is usually done by filing away a small part of the surface on an inconspicuous spot (a destructive method).